Dai Nana Maouji Jilbagias no Maou Keikoku-ki

Alexander and his allies launch a surprise attack on the Demon King’s fortress, aiming to kill him and end the war. The mission fails as the group is quickly overwhelmed by the Demon King’s strength. Alexander dies but is reborn two years later as the Demon King’s son, retaining his previous memories. He decides to use this second opportunity to dismantle the demon kingdom from within, now known as Zilbagias. He soon faces the challenge of maintaining his cover as a young demon prince, which proves more difficult than he expected.